I think Vader could definitely 'take on' Mace, although I'd love to see how that one would turn out. Due to his inability to summon Sith lightning Vader would pretty much be limited to telekinetic assaults, unless someone can provide me with another branch of offensive Force-use that he has access to. Meanwhile, Mace has shown himself to be capable of grounding a Force-grip from Kar Vastor while being hurled into tree trunks and akk hounds, so chances are he'd be able to somewhat mitigate the advantage his opponent has in terms of sheer power.
Vader's TK abilities are more then enough to deal with Mace in a force fight look how he simply mauls this Jedi in TFU:
http://www.youtube.com/watch?v=CAn00Gfg4tU&feature=related - the end of the vid
Then proceeds to do some unknown Dark Side attack that looks like a hadoken:
http://www.youtube.com/watch?v=00xbRoqVwBM - 2:16
Not to mention that devastating force wave he throws out.
And, yeah he managed to ground Vastors force grip back into the jungle...only for the jungle to attack him in turn, Vastor turned Mace into a rag-doll.
Now that Vader's been stated as being superior to Kar Vastor, if Mace had so many problems with Vastor to the point where in his head he flat out admitted even on his best day he could never beat him, imagine how Vader would maul him with the force, considering he's you know better in every respect then the guy Mace can never beat straight up.
However, Vader doesn't have much a chance at all in a lightsaber duel. Mace was fast enough to tie up Grievous with an offensive barrage, and managed to defeat Darth Sidious in personal combat. As skilled as Vader has become, he can't win against someone who is at least as proficient with the blade as he is while also having the advantage of being notably faster.
The same Grievous, that Count Dooku regularly thrashed around? The same Grievous that the movie showed Obi Wan beat rather easily. That same Count Dooku who Anakin "made a joke out of" , the same Obi Wan who barley beat a deranged, heartbroken Vader. Mace beating Grievous is NOT proof of him being superior in saber combat to Vader. As for Sidious, the whole scene where Anakin is dumbfounded by their speed is n-canon as it directly contradicts the movie in every way as Anakin didn't even come into the room till Mace had Sidious on the ground.
How many Jedi must Vader beat/kill for this "He not FAST!!" crap to be thrown out the window? How many of the Jedi he's faced have had the advantage of speed on him? Largely none, because Vader is not the slow clunky doofus we see in ANH, Vader with the enhancements of the suit is very fast and very agile, not to mention the power boost he gains from the mechanical enhancements. The only person who has outclassed Vader was the reborn Maul a pure dark side clone/apparition, and even then he wasn't out matched to the point where he was a helpless child in awe by Maul's speed. The Prophets of The Dark Side noted that they were nearly equals.
However, I can see Mace beating Vader in a saber duel but it would be because of his Vaapad (Throwing Vaders rage and such back at him) NOT his blinding speed, and the victory would only happen after a while, which I doubt Vader would even let last that long considering he has the clear advantage in the force department, and KNOWS of Mace Windu's dueling skills, why would he even bother with a saber duel?